A billboard in Stockholm's Odenplan Square has been turning heads, and it's not because of a provocative photo or a bold slogan. What makes this billboard so unique is that it coughs — and it does so every time a smoker passes by.

At first, the billboard appears normal and still, displaying a black-and-white photo of a man from the shoulders up, with white text at the bottom of the screen. But if someone is smoking nearby, the man starts to cough and gets an unpleasant expression on his face.

It's an ad for pharmacy chain Apotek Hjärtat, which advertises its stop-smoking aids after the coughing finishes. The company is trying to encourage people to quit smoking and capitalize on those who may want to kick the habit at the beginning of a new year.

An effective way to get people's attention

To accomplish the cool trick, advertising agency Akestam Holst equipped a digital screen with smoke detectors and placed it the middle of a busy pedestrian area where people tend to smoke. The agency also filmed smokers' reactions as they discovered the billboard. As you can see in the video above, they range from surprise to annoyance to amusement.

"The purpose was to drive the conversation about this topic, documenting the reactions, encouraging people to live a healthy lifestyle," Fredrik Kullberg, marketing director at Apotek Hjartat, told CNN. "The reaction has been mostly really positive."

This isn't the first time Akestam Holst has caused a stir with billboards that react to things around them. In a 2014 advertisement for hair-care products, the company equipped a digital screen on a metro platform with ultra sonic sensors so that the model's hair would blow around every time a train arrived.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.

This Swedish billboard coughs at smokers
The ad for pharmacy chain Apotek Hjärtat is equipped with smoke detectors, and the company hopes the trick will encourage people to quit smoking.